1. Field of the Invention
The present invention relates to satellite communication systems, and is more particularly related to a satellite transmission system.
2. Discussion of the Background
Modern satellite communication systems provide a pervasive and reliable infrastructure to distribute voice, data, and video signals for global exchange and broadcast of information. These satellite communication systems have emerged as a viable alternative to terrestrial communication systems. Another popular application is direct television broadcasting, which has provided a competitive alternative to cable television systems. Irrespective of the application, a satellite communication system must be designed to support the largest possible traffic capacity, which can translate to lower cost per telephone circuit, or television service, for example, and thus, to increased revenue for the system operator. The largest factors that affect capacity of a satellite communication system are the satellite transmission power and bandwidth.
To maximize traffic capacity, a typical satellite system employs multiple carriers within a single transponder, thereby requiring wide-band amplifiers to support the multiple carriers. However, wide-band amplifiers are particularly susceptible to intermodulation distortion when they are driven into saturation. As a practical consequence, the wide-band amplifier is driven below its peak power, which reduces traffic capacity. Therefore, it is observed that when signals from a large number of terminals are transmitted to a communication satellite, performance of the satellite communication system is limited by the satellite""s ability to transmit power.
Satellite communication systems also face the challenge of maintaining proper timing, as the satellite travel in geosynchronous orbit. To minimize timing errors, the signals that are transmitted from the terminals to the satellite are synchronized. Accordingly, these signals (or carriers) are received at the satellite synchronized in time; however, the total power transmitted varies significantly over the period of a symbol (i.e., the unit of time over which individual information elements are transmitted). As a result, the satellite amplifiers are driven further into saturation than the average power would imply, thereby increasing interference due to intermodulation. FIG. 1, as discussed below, illustrates the phenomenon that arises from the synchronization of numerous carriers.
FIG. 1 shows charts of the envelopes of multiple carriers and the resulting average power. Chart 101 corresponds to a carrier, f1, that is transmitted, for example, by a gateway station or a terminal. In this example, it is assumed that N carriers are received at the satellite. Graph 103 shows the envelope of the Nth carrier. It is seen in Graph 105 that the average power of all the carriers (i.e., N carriers) produces a xc2x11 dB variation; that is, a peak-to-peak variation of 2 dB. Therefore, a satellite power amplifier that handles multiple carriers experiences increased signal interference and reduced efficiency of the satellite communication system.
A drawback with the traditional approach is that the implementation of a gateway or terminal that generates synchronized signals introduces power variation to the satellite, resulting in increased signal noise, thereby decreasing traffic capacity.
Based on the foregoing, there is a clear need for improved approaches for minimizing the variation in satellite transmit power.
There is also a need to increase the efficiency of the satellite system.
There is also a need to enhance transmission capacity.
Based on the need to streamline power efficiency, an approach for generating transmission signals to provide constant satellite power is highly desirable.
According to one aspect of the invention, a method is provided for minimizing variation in transmit power in a satellite communication system. The method includes generating a plurality of synchronized signals. In addition, the method encompasses selectively offsetting transmission times of a portion of the plurality of synchronized signals. Under this approach, the traffic capacity of a satellite communication system is increased.
According to another aspect of the invention, a gateway station system for communicating with a communication satellite comprises a plurality of signal sources that are configured to generate synchronized signals. A control logic is coupled to the plurality of signal sources and is configured to offset selectively transmission times of a portion of the synchronized signals. The above arrangement advantageously provides enhanced system efficiency.
According to another aspect of the invention, a terminal apparatus for communicating with a communication satellite that processes synchronized signals comprises a receiver that is configured to receive one of the synchronized signal from the communication satellite. An offset-determination logic is coupled to the receiver and is configured to determine whether the one synchronized signal is offset. The above arrangement advantageously provides reduced intermodulation interference.
In yet another aspect of the invention, a method is provided for establishing a satellite communication channel. The method includes receiving a channel establishment request message from a terminal over a random access channel (RACH) to initiate establishment of the satellite communication channel. The method also includes generating an assignment message in response to the receiving step. The assignment message has a control flag field that indicates whether a transmission signal is offset. The method further includes transmitting the assignment message to the terminal over an access grant channel (AGCH). Under this approach, system efficiency of a satellite communication system is increased.
In yet another aspect of the invention, a computer-readable medium carrying one or more sequences of one or more instructions for establishing a satellite communication channel is disclosed. The one or more sequences of one or more instructions include instructions which, when executed by one or more processors, cause the one or more processors to perform the step of receiving a channel establishment request message from a terminal over a random access channel (RACH) to initiate establishment of the satellite communication channel. Another step includes generating an assignment message in response to the receiving step. The assignment message has a control flag field that indicates whether a transmission signal is offset. Another step includes transmitting the assignment message to the terminal over an access grant channel (AGCH). This approach advantageously minimizes variation in the average power of the synchronized signals.
In yet another aspect of the invention, a memory for storing information establishing a satellite communication channel, comprises a data structure that includes a control flag field for indicating whether a transmission signal is offset according to a predetermined distribution and an offset period. Accordingly, the above approach advantageously reduces intermodulation interference.